Controllable drivers may be used to vary the voltage supply provided to a load. One known use of such controllable drivers is to controllably dim an LED output load. In such drivers, there is typically a primary converter adapted to receive and convert an input supply in response to at least one desired supply signal from control circuitry. The said control circuitry typically generates the at least one desired supply signal from at least one input control signal. The primary converter and the output load are usually isolated from one another by a pair of magnetically coupled windings.
It is known to isolate the control circuitry, which generates at least one desired supply signal, from the primary converter with an opto-isolator. However, an opto-isolator to transfer the desired supply signal may be expensive. It is also known to provide a feedback signal from the output load to the primary converter or the control circuitry. Providing such a feedback signal allows an error signal, corresponding to the difference between desired and current supply levels, to be generated. However, the use of a feedback signal adds the possibility of electromagnetic interference test failures and potential energy surges.
FIG. 6 shows an exemplary prior art topology of a known driver 6. The controllable driver 6 comprises the primary converter 61, the control circuitry 62 and an opto-isolator 64.
The primary converter 61 receives and converts an input supply Vsup using a power generator 611 to generate a supply signal Vload for provision to a load 69. The primary converter 61 and the output load 69 are electrically isolated from another by a first winding 612 and a second winding 614, magnetically coupled together by a shared magnetic core 613. The level of the supply signal Vload is adjustable by the power generator 611 in response to an error signal, also called an adjustment signal, Vad. This adjustment signal is provided to the primary converter 61 by the control circuitry 62 via the opto-isolator 64. Comparator circuitry of the control circuitry 62 is used to generate the said adjustment signal Vad in response to a comparison performed between a desired voltage signal generated by a dimming interface and a feedback signal Vfb, directly connected to the comparator circuitry from the second winding 614 (i.e. from the isolated supply signal supplied to the load). An interface 63 generates the said desired voltage signal in response to at least one input control signal Vin.
The driver 6, therefore, delivers a controllable voltage supply signal (Vload) to the load 69, whilst ensuring isolation of the said load 69 from the primary converter 61. The control circuitry 62 used to control the voltage supply Vload is also isolated from the primary converter 61.
US20140167634A1 discloses a LED driver includes a transformer, current control loop and current adjustment circuit. The current adjustment circuit connects to either of the primary side or the secondary side directly, and uses an opto-coupler to access an PWM dimming signal.